Preparation of an end of a thermoplastics tube manufactured by spirally winding a hollow profile

ABSTRACT

A method for preparing an end of a thermoplastics tube ( 1 ) is provided, wherein the tube is manufactured by spirally winding and welding together of a hollow profile ( 1   a ) having a substantially rectangular cross section. The method includes the following steps: cutting the end of the tube by a planar transversal section; extruding a second hollow profile ( 2 ) having a rectangular cross section and an outer shape essentially similar to Fat of the hollow profile ( 1   a ) from which the tube ( 1 ) is manufactured; spirally winding the extruded second hollow profile ( 2 ) around a drum having a diameter corresponding to inner diameter of the tube ( 1 ); cutting a loop ( 2   a ) from the second hollow profile ( 2 ) in a length corresponding to circumference of the tube end; joining the ends of the loop ( 2   a ) to each other, whereby a ring ( 2   b ) is formed having the same inner and outer diameters as the spirally wound tube ( 1 ); placing the ring ( 2   b ) against the cut tube end; and welding the ring ( 2   b ) to the tube end.

This invention concerns a method for preparing a thermoplastics tubemanufactured by spirally winding and welding together of a hollowprofile having a substantially rectangular cross section.

A drawback of spirally wound tubes of the above mentioned type is thaton cutting the tube no unbroken cutting surface is received, but thecutting surface comprises only an outer and an inner edge as well as twoshort portions where the cut passes through radially directed side wallsof the hollow profile. This results in that dirt easily will penetrateinto the spirally extending cavity between the inner and outer wall ofthe tube and further, joining of these tubes will become difficult dueto relatively thin and unevenly applied material thickness in a cut tubeend.

Previously attempts have been done to overcome this, for instanceaccording to EP-A 0 540 770, by turning a groove of uniform width anddepth between the outer and the inner tube wall in an end of a spirallywound double wall tube, and then heating the plastics material in thetube walls defining the turned groove. A specially made mould sealingring is possibly inserted in the turned groove, and then the heated tubeend with or without said sealing ring is inserted into a conicallytapering groove in an annular mould, whereby the tube end will get awished shape. If no sealing ring is used the tube end must, aftercooling, still be provided with a welding seam closing a narrow gapbetween the outer and inner tube walls, which gap will remain when thetube end has been moulded. This previously known method is primarilyintended to mould and prepare a male and a female end of a socket pipe.A drawback of the method is that it requires a lot of extra measures inthe tube end and comprises a lot of time consuming working steps as wellas specially made accessories.

A considerably simplified method is described in the Finnish patentapplication No. 964518, in which a cut tube end is heated until theplastics material is changed into a plastic condition, and then the tubeend is inserted into an annular groove in a press mould corresponding tothe cross section of the tube and is exerted to an axially directedpushing force. The plastics material in the end of the tube is herebypressed together so that a continuous planar end face is achieved.According to another embodiment a string of molten plastics mass isextruded onto the bottom of the annular groove before the tube end isinserted. This method is relatively suitable for spirally wound tubeshaving small diameters, but when the dimensions of the tubes increasethis method will be too clumsy.

The object of the present invention is to provide a method which is assuitable for large as for small spirally wound tubes, and which willresult in tube ends which may be joined together according to samemethods as solid tubes. This is achieved by a method, which ischaracterized by the steps as follow:

cutting the end of the tube by a planar transverse section;

extruding a second hollow profile having a rectangular cross section andan outer shape essentially similar to that of the hollow profile fromwhich the tube is manufactured;

spirally winding the extruded second hollow profile around a drum havinga diameter corresponding to inner diameter of the tube;

cutting a loop from the second hollow profile in a length correspondingto circumference of the tube end;

joining the ends of the loop to each other, whereby a ring is achievedhaving same inner and outer diameters as the spirally wound tube;

placing the ring against the cut tube end, and

welding the ring to the tube end.

The second hollow profile is preferably manufactured with a somewhatlarger wall thickness than the hollow profile of the tube itself. Intubes produced from small profiles the second profile is thereforesuitably extruded as a solid profile without cavity.

Further features of the invention will appear from the dependent claims3 to 6.

Below the invention will be described in more detail with reference tothe enclosed drawing, in which

FIG. 1 shows schematically a longitudinal section through the tube wallin the end of a tube, the end of which is prepared according to theinvention,

FIG. 2 shows a longitudinal section through the tube wall in the end ofanother tube, the end of which is prepared according to the invention,

FIG. 3 shows a further example of a longitudinal section through thetube wall in the end of a tube, end prepared according to the invention,

FIGS. 4a and 4 b show schematically an axial view and a side view,respectively, of the extruded and spirally wound second hollow profilefor the end preparation,

FIG. 5 shows a loop cut from the spirally wound second hollow profile inFIGS. 4a and 4 b and

FIG. 6 shows the loop in FIG. 5 welded together to an end ring.

According to the invention the end preparation of a thermoplastics tube1, produced by spirally winding and welding together of hollow profile 1a having a substantially rectangular cross section, is carried out byinitially cutting the end of the tube 1 in a planar transverse section.A second hollow profile 2, having a rectangular cross section and anouter shape substantially similar to that of the hollow profile 1 a fromwhich the tube 1 is produced, is extruded and spirally wound around adrum having a diameter corresponding to the inner diameter of the tube1. No welding together of adjacent windings is carried out, but thesecond hollow profile 2 takes a shape mainly corresponding to a spring,as shown in FIG. 4b. For each tube end a loop 2 a is then cut from thespirally wound second hollow profile 2, in a length corresponding tocircumference of the tube 1 or tube end (FIG. 5). Both ends of the loop2 a are then welded together so that a ring 2 b is formed (see FIG. 6).having an outer and an inner diameter equal to those of the tube 1itself The ring is then placed against the cut tube end and is welded toit by an inner and an outer weld seam 3 a, 3 b, whereby an impact proofand closed tube end is formed. The closed tube end can be joined toanother similar tube end according to same methods as those being usedin joining solid tubes, i.e. by stub welding, electro welding using adisk like welding ring, by extrusion welding or a sleeve joint.

When the tube ends are intended to be joined together by means of asleeve joint the end of one tube will be provided with a socket 4 in away known as such and the tube profile 2 for the end of the other tubeis extruded through a nozzle arranged to produce longitudinal grooves 5in that side wall 2 c of the hollow profile 2, which during thesubsequent spirally winding is positioned outermost forming an outermantle of the coil and hence of the ring 2 b, too. An elastic sealingring 6, such as an O-ring or a lip like ring can then be positioned intothe groove 5, before the tube end is pushed into the socket 4. This isschematically shown in FIG. 3.

In order to receive as good strength as possible in the pipe end, thesecond hollow profile 2 is preferably extruded with a somewhat largerwall thickness than the hollow profile from which the tube 1 itself isproduced. When the tube is built up of small hollow profiles this willresult in that the cavity of the second hollow profile 2 will completelydisappear and a solid profile is received, as schematically shown inFIG. 2.

The most elegant way to join the ends of a loop 2 a cut from the secondhollow profile 2 is by stub welding, while the most simple way to weldthe thus received ring 2 b to the tube end is by means of extrusionwelding.

What is claimed is:
 1. A method for preparing an end of a thermoplasticstube manufactured by spirally winding and welding together of a firsthollow profile having a substantially rectangular cross section,comprising: cutting the end of the tube by a planar transverse section;extruding a second hollow profile having a rectangular cross section andan outer shape essentially similar to that of the first hollow profile;spirally winding the extruded second hollow profile around a drum havinga diameter corresponding to inner diameter of the tube; cutting a loopfrom the second hollow profile in a length corresponding to acircumference of the end of the tube; joining ends of the loop to eachother, whereby a ring is formed having inner and outer diameters equalto inner and outer diameters of the spirally wound tube; placing thering against the cut end of the tube; and welding the ring to the end ofthe tube.
 2. A method according to claim 1, wherein the second hollowprofile is extruded with a larger wall thickness than the hollow profilefor the tube itself.
 3. A method according to claim 1, wherein the endsof the cut loop are joined together by stub welding.
 4. A methodaccording to claim 1, wherein the ring is welded to the end of the tubeby extrusion welding.
 5. A method according to claim 1, wherein a sidewall of the second hollow profile is shaped during extrusion to includea longitudinal groove, and wherein the side wall forms an outer mantleof the end of the tube.
 6. A method for preparing an end of athermoplastics tube manufactured by spirally winding and weldingtogether of a first hollow profile having a substantially rectangularcross section, comprising: cutting the end of the tube by a planartransverse section; extruding a second hollow profile having arectangular cross section and an outer shape essentially similar to thatof the first hollow profile, wherein the second hollow profile has awall thickness that is greater than a wall thickness of the first hollowprofile; spirally winding the extruded second hollow profile around adrum having a diameter corresponding to inner diameter of the tube;cutting a loop from the second hollow profile in a length correspondingto a circumference of the end of the tube; joining ends of the loop toeach other, whereby a ring is formed having inner and outer diametersequal to inner and outer diameters of the spirally wound tube; placingthe ring against the cut end of the tube; and welding the ring to theend of the tube so as to convert the second hollow profile into a solidprofile.